This invention relates to an improvement of a piezoelectric type pressure indicator utilized to measure the pressure in a cylinder of an internal combustion engine, that is, the pressure diagram, and more particularly a piezoelectric type pressure indicator wherein an electric voltage of a piezoelectric element generated by inertia due to accleration is corrected for improving the measuring accuracy.
FIG. 1 shows a basic construction of a prior art piezoelectric pressure indicator provided with a conventional acceleration compensation mechanism. The pressure indicator shown in FIG. 1 comprises a cylindrical casing or sleeve 1 made of metal such as stainless steel, and a plate shaped pressure receiving member 2 of the same metal is mounted on the inner periphery of the bottom 1a of the casing 1. A cylindrical insulating member 3 made of Teflon, for example, is inserted in the casing, and three piezoelectric elements 4-1 to 4-3 made of X-cut quartz are laminated on the pressure receiving member 2 inside of the insulating member 3. Electrodes 4a to 4d are applied to the piezoelectric elements 4-1 to 4-3 and interconnected such that the electromotive forces or voltages generated therein when they are pressed in the direction of X add each other. Between the insulating member 3 and the piezoelectric elements 4-1 to 4-3 is formed a gap A for improving the measuring accuracy. A weight 5 (also called a mass) made of tungsten and adapted to compensate for acceleration and also acting as an electrode memeber is mounted on the electrode 4d of the laminated piezoelectric elements 4-1 to 4-3. A connecting piece 7 with a lead wire 10 is disposed in a recess 6 at the center of the upper surface of the weight 5. An acceleration compensating piezoelectric element 8 made of an X- cut quartz crystal and having a perforation 9 through which the lead wire 10 passes is mounted on the weight 5. A bushing 11 made of stainless steel is inserted into an upper end of the casing 1 to overlie the piezoelectric element 8 so as to derive out an electric signal corresponding to the pressure applied to the pressure receiving member 2 through lead wire 10. The lead wire 10 is connected to a terminal 13 within a central opening 12 of the bushing member 11 serving as an output voltage deriving means. The terminal 13 is supported by a sealing member 14 which hermetically seals the central opening 12. The piezoelectric element 8 is provided with electrodes 8a and 8b on both surfaces thereof.
The operation of the piezoelectric pressure indicator described above will be described with reference to a model shown in FIG. 2 in which mM represents the mass of a vibratory portion acting as the pressure receiving member 2 subjected to pressure P to be measured, mD represents the mass of a pressure transmitting member, ms represents the mass of the acceleration compensating weight 5, and the electrodes 4a, 4c and 8a of the piezoelectric elements 4-1, 4-2, 4-3 and 8 are connected in parallel to ground. Other electrodes 4b, 4d and 8b are also connected in parallel to the lead wire 10. Symbols (+) and (-) show the polarities of the electromotive forces appearing at the electrodes 4a to 4d, 8a and 8b when a pressure is applied in the direction of X axis. Denoting the piezoelectric constant of the piezoelectric elements 4-1 to 4-3 and 8 by d.sub.11 and the acceleration applied to the pressure indicator by .alpha., the electromotive force caused by the acceleration .alpha., that is, voltage Q appearing at the electrodes 4b and 4d of the piezoelectric element is expressed by the following equation. ##EQU1## where F represents the inertia exerted on the piezoelectric elements 4-1 to 4-3 due to the acceleration .alpha.. Denoting the inertia exerted on the piezoelectric element 8 due to the acceleration .alpha. by F*, the electromotive force Q* generated at the electrode 8b of the piezoelectric element 8 owing to the mass mS of the acceleration compensating weight 5 is expressed by the following equation. ##EQU2## However, in order to compensate for the electromotive .alpha. force generated by the inertia due to the acceleration .alpha., it is necessary to satisfy the following equation EQU Q+Q*=0 (3).
Then the electromotive force caused in response to the acceleration .alpha. would be cancelled, thereby correcting the effect of acceleration. More particularly, equations (1) and (2) are put into equation (3) to obtain an equation, ##EQU3## From equation (4), we can obtain the following equation EQU mS=2 (mM+mD) (5).
As described above, by compensating for the electromotive force generated in response to the inertia due to the acceleration applied to the pressure indicator, pressure can be measured with high accuracies.
In the prior art pressure indicator described above, since the lead wire 10 is connected to the inside of the acceleration compensating weight 5, the connection of the lead wire can be made rigid and reliable. However, since only one acceleration compensating quartz piezoelectric element 8 having a perforation 9 is used, it is impossible to obtain sufficient compensation. For this reason, it has been proposed to use three perforated acceleration compensating piezoelectric elements 8-1 to 8-3 which are laminated as shown in FIG. 3 in which elements corresponding to those shown in FIG. 1 are designated by the same reference numerals. In FIG. 3, five piezoelectric elements 4-1 to 4-5 are laminated on the pressure receiving member 2 and these piezoelectric elements 4-1 to 4-5 are interconnected with the perforated piezoelectric elements 8-1 to 8-3 to cancel the electromotive force caused by acceleration. With this construction, however, since three perforated acceleration compensating piezoelectric elements 8-1 to 8-3 are used each having a manufacturing cost which is about 10 times that of the imperforate piezoelectric element, the cost of the pressure indicator increases. Moreover, since the lead wire 10 is connected to the acceleration compensating weight 5, the connection of the lead wire becomes difficult depending upon the position of the weight 5.
The piezoelectric pressure indicator of the type described above is mounted such that its pressure receiving member projects to a point near the wall surface of the engine combustion chamber. Therefore, the pressure receiving member is usually made of heat and corrosion resistant metal. However, in a high temperature corrosive atmosphere, corrosion and degradation of the pressure receiving member is unavoidable. To eliminate this problem, that is, for the purpose of protecting the pressure receiving member, it has been tried to mount the pressure receiving member through a metal rod. With this method of mounting, however, sufficient heat resistant property can not be provided and hysteresis is caused by the elasticity of the metal rod.
In the piezoelectric pressure indicator utilizing a plurality of piezoelectic elements, since the voltage produced by each element is very small, for the purpose of preventing leakage of the voltage and degradation of the elements, it is usual to sufficiently remove moisture remaining in the casing 1 by drying or to assemble the pressure indicator in an environment containing very small quantities of moisture. However, as the uppermost piezoelectric element 8 and the bushing member 11 inserted in the upper portion of the casing contact directly, the internal air is difficult to discharge and it takes a long time for drying.